1. Field of Invention
The present invention relates to piezoelectric devices, such as piezoelectric resonators and piezoelectric oscillators for use in various electronic devices, and piezoelectric vibrating gyroscopes for use as angular speed sensors; More particularly, the invention relates to a tuning-fork piezoelectric resonator element used in those devices, and a production method of the tuning-fork piezoelectric resonator element.
2. Description of Related Art
In related art consumer and industrial electronic devices such as timepieces, household electrical appliances, various information and communication devices, and office automation devices, piezoelectric devices, such as a piezoelectric resonator, an oscillator and a real time clock module in which a piezoelectric resonator and an IC chip are sealed in the same package, can be used as a clock source of an electronic circuit. Furthermore, piezoelectric vibrating gyroscopes can be used as rotation angular speed sensors to control the attitude and navigation of ships, aircrafts, automobiles, and the like, and to prevent and detect the camera shaking of video cameras and the like, and can also be applied to rotating direction sensors, such as three-dimensional mice. Such a piezoelectric vibrating gyroscope is disclosed in, for example, Japanese Unexamined Patent Application Publication No. 7-55479 (JP 479), and Japanese Unexamined Patent Application Publication No. 10-170272 (JP 272).
In particular, with a reduction in the related art in size and thickness of electronic devices in which these piezoelectric devices are mounted, the piezoelectric devices are required to further reduce the size and thickness thereof. The piezoelectric devices are also required to ensure a low CI (crystal impedance) value and to achieve high quality and high stability. In order to keep the CI value low, for example, a tuning-fork piezoelectric resonator element provided with resonating arms each having a groove has been developed, for example, as disclosed in Japanese Unexamined Patent Application Publication No. 56-65517 (JP 517), and the specification of Japanese Patent Application No. 2000-595424 (JP 424).
In this tuning-fork piezoelectric resonator element provided with resonating arms each having a groove, upper and lower principal surfaces 3a and 3b of a pair of resonating arms 2a and 2b extending in parallel from a base portion 1 are provided with linear grooves 4a and 4b extending along the lengthwise direction thereof, as illustrated in FIG. 8. As shown in FIGS. 9(A) and 9(B), first electrodes 5a and 5b are formed on side faces and bottom faces of the grooves 4a and 4b, and second electrodes 6a and 6b are formed on side faces of the resonating arms 2a and 2b. The first electrodes 5a (5b) of one of the resonating arms are electrically connected to the second electrodes 6b (6a) of the other resonating arm, thereby constituting driving electrodes to vibrate the tuning-fork quartz resonator element. When an alternating voltage is applied from connecting terminals 7 to the driving electrodes, electric fields E1 and E2 parallel to the principal surfaces are produced between the first electrodes 5a and 5b and the second electrodes 6a and 6b adjoining each other, and as a result, field efficiency is substantially enhanced, and the CI value can be reduced.
Usually, by processing a wafer made of a piezoelectric single-crystal material, such as quartz crystal, by wet etching using photolithography to form a desired outline of the resonator element and the grooves 4a and 4b, electrode films are formed on the surfaces of the resonator element and the grooves 4a and 4b by patterning. More specifically, corrosion-resistant films are formed on both surfaces of a quartz wafer, and photoresists are applied thereon and are dried to form resist films. In a state in which a pair of upper and lower first photomasks having the same etching pattern corresponding to the desired outline of the resonator element are placed thereon, the surfaces of the corrosion-resistant films are exposed by exposure and development, and are removed with etchant to expose the surfaces of the quartz wafer. After the remaining resist films are stripped off, photoresists are applied again on the remaining corrosion-resistant films, and are dried to form new resist films. Then, a pair of upper and lower second photomasks having an etching pattern corresponding to the shape of the grooves of the resonating arms are placed thereon, and the surfaces of the corrosion-resistant films are exposed by exposure and development.
Subsequently, the exposed surfaces of the quartz wafer are etched with a quartz etchant, thereby forming the outline of the resonator element including the resonating arms. Furthermore, the exposed surfaces of the corrosion-resistant films are removed with etchant to expose the surfaces of the quartz wafer. By half-etching the exposed surfaces of the quartz wafer to a predetermined depth with a quartz etchant, grooves are formed on the upper and lower principal surfaces of the resonating arms. An electrode material is deposited on all the surfaces of the quartz element thus formed, including the inner faces of the grooves of the resonator element, by evaporation, sputtering, and the like, and is polarized by photoetching, thereby forming desired driving electrodes, extraction electrodes, and lines.
For example, the used quartz wafer is formed by cutting out quartz crystal around the X-axis at a cutting angle xcex8 ranging, for example, from approximately 30 minutes to 2 degrees from the Z-axis. As shown in FIG. 8, the lengthwise, widthwise, thickness directions of the resonating arms 2a and 2b of the tuning-fork piezoelectric resonator element are oriented corresponding to the Y-axis called a mechanical axis of the quartz crystal structure, the X-axis called an electric axis, and the Z-axis called an optical axis, respectively. Therefore, the widthwise direction of the resonating arms 2a and 2b coincides with the X-axis direction, the lengthwise direction corresponds to the Yxe2x80x2-direction inclined at the angle xcex8 to the Y-axis direction, and the thickness direction corresponds to the Zxe2x80x2-direction inclined at the angle xcex8 to the Z-axis direction.
As shown in FIG. 9(A), the grooves 4a (4b) of the resonating arm 2a (2b) are placed so that a center line C1 thereof coincides with a center line C2 of the resonating arm. However, since most piezoelectric resonator elements are made of a piezoelectric single-crystal material having etching anisotropy, such as quartz crystal, the cross-sections of the resonating arm and the grooves formed by wet etching are often asymmetrical with respect to the center lines C1 and C2 because of the crystal orientation. In particular, the etching rate of the quartz crystal has a crystal-axis dependence, and the quartz crystal is prone to be etched in widthwise direction of the resonating arm, that is, in the +X direction in the example shown in FIG. 8. Therefore, the cross-sections of the grooves 4a (4b) are not shaped like an ideal rectangle that is shown by imaginary lines 8 in FIG. 9(B), but are asymmetrical, that is, the left side faces thereof are inclined rightward in the figure, and a projection 9 is formed on the right side face of the resonating arm 2a (2b).
For this reason, a non-negligible difference in stiffness is formed between the right and left sides of the center line C2 of the resonating arm, and bending of the resonating arm is unbalanced between the right and left sides, that is, inside and outside of the tuning fork during excitation. Consequently, vibrations are not confined in the resonating arm, but leak from a mount portion of the piezoelectric resonator element toward the package, which may cause a loss of strain energy. When the amount of unbalance in bending between the right and left sides of the resonating arm increases and the loss of strain energy of the piezoelectric resonator clement increases, the natural frequency, that is, the oscillation frequency decreases, and the natural frequency varies widely.
In a case in which the first photomasks are incorrectly aligned on the upper and lower surfaces of the quartz wafer during the above process of forming the outline of the piezoelectric resonator element, the upper principal surface 3a and the lower principal surface 3b of the resonating arm 2a (2b) are offset from each other in the widthwise direction, as shown by the center lines C1 and C2 in FIG. 10, and the cross-section of the resonating arm may be vertically asymmetrical in the thickness direction. In particular, when size reduction of the piezoelectric resonator element is furthered, since positioning of the photomasks becomes more difficult, and the positioning accuracy is decreased, the cross-section tends to be asymmetrical.
In an elastic member having such an asymmetrical cross-section, tensile force and compressive force produced by the inverse piezoelectric effect of the electric fields E1 and E2 act vertically in an unbalanced manner in the thickness direction (Zxe2x80x2-direction) of the cross-section taken along the widthwise direction perpendicular to the principal surfaces, that is, along the X-direction, as shown in FIG. 10. For this reason, the resonating arms 2a and 2b receive a moment that twists the entire resonating arms 2a and 2b in the thickness direction during excitation, and exhibit flexural vibration in the widthwise direction while being displaced in the thickness direction. As a result, vibrations may leak to lose strain energy, and vibration characteristics may be unstable. When the loss of the strain energy is increased, the natural frequency, that is, the oscillation frequency of the piezoelectric resonator element decreases, and the natural frequency varies widely.
The present invention addresses or solves the above and/or other problems, and provides a tuning-fork piezoelectric resonator element which is made of a piezoelectric material having etching anisotropy, which has resonating arms each having a groove, and in which stable bending motion is ensured by achieving a good balance of stiffness between the right and left sides of the resonating arms in the widthwise direction or enhancing the balance, and high stability is achieved in addition to enhancement of performance by reducing the C1 value.
The present invention also provides a tuning-fork piezoelectric resonator element having grooved resonating arms in which, performance is enhanced by reducing the C1 value, the displacement in the thickness direction of bending vibration of the resonating arms caused by the offset between the upper and lower principal surfaces of the resonating arms is effectively resolved or reduced, the loss of strain energy due to vibration leakage is prevented or reduced, and stable vibration characteristics are ensured.
The present invention also provides a production method for such a tuning-fork piezoelectric resonator element.
The present invention also provides high-performance and high-stability piezoelectric device using such a tuning-fork piezoelectric resonator element.
In order to address or achieve the above, according to a first aspect of the present invention, there is provided a tuning-fork piezoelectric resonator element made of a piezoelectric material having etching anisotropy in a predetermined direction, and including a pair of resonating arms extending from a base portion, and a driving electrode including first electrodes provided on front and back principal surfaces of each of the resonating arms, and second electrodes provided on side faces of the resonating arm. One of the principal surfaces of the resonating arm is provided with a groove extending in the lengthwise direction of the resonating arm so that a center line of the groove is offset from a center line of the resonating arm in a direction opposite to the predetermined direction of the etching anisotropy. The first electrode provided on at least one of the principal surfaces is formed of an electrode film formed on a side face of the groove.
When the outline of the tuning-fork piezoelectric resonator element and the grooves of the resonating arms are formed by subjecting a piezoelectric material having etching anisotropy to a related art wet etching method, as described above, there is a danger that the cross-section will be asymmetrical in the widthwise direction and that the stiffness of each resonating arm will increase from its center line in the direction opposite to the predetermined direction of the etching anisotropy. By arranging the groove of the resonating arm, as in the present invention, the unbalance of the stiffness due to such an asymmetrical cross-section can be prevented or enhanced. Therefore, bending of the right and left sides of the resonating arm is stabilized, a loss of strain energy due to vibration leakage is prevented, and stable bending can be repeated.
Quartz crystal that has been adopted in the related art is preferable as the piezoelectric material. In this case, by placing the lengthwise, widthwise, and thickness directions of the resonating arm corresponding to the Y-axis, X-axis, and Z-axis directions, respectively, of the quartz crystal, and placing the groove offset from the center line of the resonating arm in the widthwise direction, that is, in the xe2x88x92X-direction of the quartz crystal, since the etching rate of the quartz crystal is high in the +X-direction, a good balance of stiffness can be ensured between the right and left sides of the resonating arm, in spite of the asymmetrical cross-sectional shape.
In an exemplary embodiment, it is confirmed that, when the offset amount of the center line of the groove from the center line of the resonating arm is within the range of 1% to 5% of the width of the resonating arm, more preferably, within the range of 2% to 4%, the oscillation frequency of the tuning-fork quartz resonator element can be increased, and does not widely vary even when the position of the groove is shifted in the widthwise direction of the resonating arm within the range of manufacturing errors.
According to a second aspect of the present invention, there is provided a tuning-fork piezoelectric resonator element including a pair of resonating arms extending from a base portion, grooves extending on upper and lower principal surfaces of each of the resonating arms in the lengthwise direction, and driving electrodes composed of a first electrode provided on a side face of each of the grooves and a second electrode provided on a side face of each of the resonating arms. One of the grooves is offset from the other groove in the widthwise direction of the resonating arm in the same direction as the direction in which the principal surface having the one of the grooves is offset from the principal surface having the other groove. The centers of gravity of two upper and lower portions of the widthwise cross-section of the resonating arm divided into two equally in the thickness direction are aligned with the same line perpendicular to the principal surfaces. In this case, it is also preferable that the piezoelectric material be quartz crystal that is adopted in the related art, and that the lengthwise, widthwise, and thickness directions of the resonating arm are oriented corresponding to the Y-axis, X-axis, and Z-axis directions of the quartz crystal.
In this configuration, even when the upper and lower principal surfaces of the resonating arm are offset from each other in the widthwise direction, a moment due to the unbalance of tensile force and compressive force in the two upper and lower sections of the entire cross-section of the resonating arm divided into two equally in the thickness direction is cancelled, and the displacement in the thickness direction during excitation can be overcome or reduced. Therefore, the loss of strain energy due to vibration leakage is prevented, and the resonating arm can stably repeat flexural vibration.
In another exemplary embodiment, the tuning-fork piezoelectric resonator element may further include another pair of resonating arms extending from the base portion in a direction opposite to the above pair of resonating arms, and may be used in the piezoelectric vibrating gyroscope disclosed in JP 479 and JP 272 described above.
According to another aspect of the present invention, there is provided a piezoelectric device including the above-described tuning-fork piezoelectric resonator element of the present invention, and a package in which the tuning-fork piezoelectric resonator element is fixed at the base portion and is sealed. Furthermore, the present invention provides a piezoelectric device further including an IC element mounted in the package.
According to a further aspect of the present invention, there is provided a production method of a tuning-fork piezoelectric resonator element, the method including: of processing a wafer made of a piezoelectric material having etching anisotropy in a predetermined direction to form the outline of a tuning-fork piezoelectric resonator element including a base portion and a pair of resonating arms extending from the base portion, wet-etching at least one of front and back principal surfaces of each of the resonating arms to form a groove extending in the lengthwise direction of the resonating arm so that a center line of the groove is offset in the widthwise direction from a center line of the resonating arm in a direction opposite to the predetermined direction of the etching anisotropy, and forming a driving electrode by forming and patterning an electrode film on the principal surfaces and side faces of the resonating arm and on an inner face of the groove.
This makes it possible to produce, according to the related art processes, a tuning-fork piezoelectric resonator element in which the unbalance of stiffness due to the asymmetrical cross-sectional shape of the resonating arm with the groove is prevented or reduced, bending on the right and left sides of the resonating arm is stabilized, strain energy is prevented from being lost by vibration leakage, and stable bending can be repeated.
In an exemplary embodiment, quartz crystal that is used in the related art may be used as the piezoelectric material. In this case, when the lengthwise, widthwise, and thickness directions of the resonating arm are oriented corresponding to the Y-axis, X-axis, and Z-axis directions, respectively, of the quartz crystal, and the groove is offset from the center line of the resonating arm in the widthwise direction, that is, in the xe2x88x92X-direction of the quartz crystal, since the etching rate of the quartz crystal is high in the +X-direction, a good balance of stiffness can be ensured between the right and left sides of the asymmetrical cross-section of the resonating arm.
In another exemplary embodiment, it is preferable that the offset amount of the center line of the groove from the center line of the resonating arm be set within the range of 1% to 5% of the width of the resonating arm, more preferably, within the range of 2% to 4%, because the oscillation frequency of the tuning-fork quartz resonator element can be increased, and hardly varies even when the position of the groove is shifted in the widthwise direction of the resonating arm within the range of manufacturing errors.
According to a further aspect of the present invention, there is provided a production method of a tuning-fork piezoelectric resonator element, the method including: processing a wafer made of a piezoelectric material to form the outline of a tuning-fork piezoelectric resonator element including a base portion and a pair of resonating arms extending from the base portion, forming, on upper and lower principal surfaces of each of the resonating arms, grooves extending in the lengthwise direction of the resonating arm by photoetching, and forming and patterning an electrode film on the principal surfaces and side faces of the resonator element and on inner faces of the grooves. Corresponding to the offset of one of the principal surfaces from the other principal surface in the widthwise direction of the resonating arm caused in the processing step of forming the outline of the resonator element, one of the grooves to be formed on the one principal surface is offset from the other groove to be formed on the other principal surface in the same direction as the direction of the offset between the principal surfaces during the step of forming the grooves by photoetching. The centers of gravity of upper and lower portions of the widthwise cross-section of the resonating arm divided into two equally in the thickness direction are aligned on the same line perpendicular to the principal surfaces. In this case, it is also preferable that quartz crystal that is adopted in the related art be used as the piezoelectric material, and that the lengthwise, widthwise, and thickness directions of the resonating arms are oriented corresponding to the Y-axis, X-axis, and Z-axis direction of the quartz crystal.
By adding the step of adjusting the position of the groove formed on one of the principal surfaces of the resonating arm to the production process of the tuning-fork piezoelectric resonator element, it is possible to quickly cope with the offset in the widthwise direction between the upper and lower principal surfaces of the resonating arm that may be caused when forming the outline of the resonating arm, and to prevent or reduce a resultant displacement in the thickness direction. For this reason, it is possible to produce a tuning-fork piezoelectric resonator element which prevents loss of strain energy due to vibration leakage and which exhibits stable vibration characteristics, according to the related art process.
In an exemplary embodiment, the positions of the grooves can be relatively easily adjusted by aligning one of the grooves with the widthwise center of the principal surface on which the one groove is to be formed and by positioning the other groove in conjunction with the one groove.
In the production method for the tuning-fork piezoelectric resonator element of the present invention, by performing the processing step so that the tuning-fork piezoelectric resonator element further includes another pair of resonating arms extending from the base portion in a direction opposite to the pair of resonating arms, a tuning-fork piezoelectric resonator element can be obtained which is suitable for use in the above-described piezoelectric vibrating gyroscope disclosed in JP 479 and JP 272 described above.